Method for manufacturing a shadow mask

ABSTRACT

In this invention, a sheet steel is annealed and is subjected to skin-pass rolling of a reduction of 0.5-15% before forming a desired pattern of small holes therein with an etching (i.e. photoetching) method, thereby to provide a sheet steel having no elongation of the yield point and being slow in aging and to decrease the thermal deformation of the shadow mask by applying the residual stress occurring therein due to the skin-pass rolling.

Uited States Patent [191 Sato et a1.

[ METHOD FOR MANUFACTURING A SHADOW MASK [75] Inventors: Kazuo Sato, Tokyo; Soji Takahashi,

Hachioji, both of Japan [73] Assignee: Hitachi, Ltd., Japan [22] 'Filed: Feb. 21, 1974 [21] Appl. No.: 444,569

[30] Foreign Application Priority Data Feb. 21, 1973 Japan 48-20167 July 4, 1973 Japan 48-75358 [52] US. Cl 29/557; 29/1635 R; 29/558; 156/16; 156/18; 313/402 [51] Int. Cl. B231 13/04 [58] Field of Search 29/557, 558, 163.5 R; 148/12 R, 11.5 R; 72/365, 366; 156/16, 8,

[56] References Cited UNITED STATES PATENTS 3,519,869 7/1970 Kuniyoshi 313/402 Uct.7,1975

3,676,914 7/1972 Fiore 156/18 X 3,707,640 12/1972 Lerner. 313/402 3,808,071 4/1974 Lernerm. 156/16 3,809,945 5/1974 Roeder 313/85 S OTHER PUBLlCATlONS The Making, Shaping and Treating of Steel, U.S.S. Eighth Edition, 1964, pp. 920, 921, 928.

Primary ExaminerVictor A. DiPalma Attorney, Agent, or Firm-Craig & Antonelli [57] ABSTRACT In this invention, a sheet steel is annealed and is subjected to skin-pass rolling of a reduction of 0.515% before forming a desired pattern of small holes therein with an etching (i.e. photoetching) method, thereby to provide a sheet steel having no elongation of the yield point and being slow in aging and to decrease the thermal deformation of the shadow mask by applying the residual stress occurring therein due to the skin-pass rolling.

11 Claims, 8 Drawing Figures FIG. I ROLLING ANNEALING SKIN PAS @LING ETCHING R ESS FORMING TENSION 0 COMPRE55|0N 3a 51 'Q E 5 Y FIG. 3b

US. Patent Oct. 7,1975 Sheet 2 of 3 3,909,928

66666660 Nwmq-mN x) seams METHOD FOR MANUFACTURING A SHADOW MASK BACKGROUND OF THE INVENTION This invention relates to improvements in a method for manufacturing a shadow mask, and more particularly to a method for manufacturing a shadow mask of a Braun tube for color TV.

A shadow mask has heretofore been manufactured by employing as a material a low carbon steel whose content of carbon is not greater than 0.1% and by rolling the material to athickness'of or below approximately 0.2 mm., thereafter forming holes in the steel sheet with photoetching (in, for example, the shadow mask of a color TV Braun tube, several hundred thousands of holes are arrayed under a predetermined pattern in case of the inch tube) and press-forming the steel sheet into the shadow mask. In general, sheet steel subjected to a high degree of cold rolling down to the thickness of or below 0.2 mm. does not have enough ductility to withstand the press forming. Even if the forming is possible, the amount of spring-back after the forming is large on accountof a high flow stress. The shape of the product differs much from the punch profile, and the forming precision is inferior. Accordingly, in order to lower the flow stress and to increase the ductility, the sheet steel is usually annealed. Since the annealed sheet steel has a yield point elongation of several percent, small wrinkles 'or lines termed the stretcher strain arise at the press-forming. In order to prevent the stretcher strain and to erase the wave in the sheet steel'caused by the annealing, the sheet steel after the annealing needs to be further treated by a'leveler. I i

In the prior-art process as stated above, some of the following disadvantages are involved. First, unless the sheet steel treated by the leveler is press-formed within several days, aging proceeds and the stretcher strain arises again. Secondly, clue to the two steps of annealing and leveling, the pattern of the'holes in the mask surface formed by the etching is distorted, and a uniform pattern is spoiled. Further, since the shadow mask is very thin as apparent from the manufacturing process, the shadow mask surface undergoes the phenomenon of locally inflating or distorting in some pictures of the television due to thermal deformation ascribable to the irradiation by'an electronbeam. This phenomenon is the cause of color shading or deflection.

SUMMARY OF THE INVENTION A first object of this invention is to eliminate the above-mentioned disadvantages and to obtain a material which does not distort the pattern of the'holes in the mask surface and which is difficult to induce the shadow mask surface, the deformation of the sheet plane is small.

In order to accomplish these various objects, this invention anneals' a startingmaterial of sheet steel and subjects the annealed material to the skin-pass rolling BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a process chart showing a method according to this invention for manufacturing a shadow mask;

FIG. 2 shows stress strain diagrams in the respective steps of this invention;

FIGS. 3a-3d are diagrams showing the residual stress distributions of a material in the respective steps of this invention;

FIG. 4 is a diagram showing the relationship between the skin-pass reduction in height and the yield point elongation of the material; and

FIG. 5 is a diagram showing the relationship between the skin-pass reduction in height and the uniform elongation as well as the total elongation of the material.

DESCRIPTION OF THE PREFERRED EMBODIMENTS FIG. 1 is a process ehartwhic'h shows the method according to this'invention for manufacturing a shadow mask. After the rolling and before the etching perforation, the material is annealed and is further subjected to the skin-pass rolling. Thus, the material having quite no yield point elongation and being slow in aging is obtained, and the working process as shown in this figure is made possible. It is also possible to decrease the amount of thermal deformation of the shadow mask by utilizing a residual stress within the material as attributed to the performance of the skin-pass rolling.

A working process for manufacturing a, mask of a sheet thickness of 0.150 mm. is taken as an example of this invention, and will be described in regular sequence.

FIG. 2 shows'the stress-strain diagrams of a material in the respective steps of this invention.

First, low carbon steel or mild steel (which may also be industrial pure iron) having a carbon content of 0.060.08%'by weight and being a starting material is subjected to a cold rolling operation, to obtain sheet steel which is 0.152 mm. thick. The stress-strain diagram of the material in this state is illustrated at A in FIG. 2. The property of the sheet steel in this state-is the same as that of sheet steel after the rolling opera tion in the prior art.

Subsequently, annealing of the sheet steel at approximately 600C. for 20 minutes is carried out under an inert gas, e.g. N or H By this step, the sheet steel becomes a material which is large in elongation as shown at Bin FIG. 2. As compared with a material having undergone the annealing step of the prior art at above 900C, the material in theinvention isfree from the wave formation in the sheet plane and is good in flatness, but it has a large yield point elongation.

though cold rolling is usually employed, hot rolling is not objectionable). Owing to this step, there is achieved a material which is quite free from the yield point elongation and large in ductility as seen at C in FIG. 2. This material is more excellent in its yield point elongation and ductility than a material after the leveling step of the prior-art process.

The material is subsequently formed with holes by etching in the general procedure conventionally used in the production of shadow masks. The perforated material is press-formed as it is (although the technique known as cold forming is usually adopted to effect press-forming, the use of the technique known as hot forming is not objectionable). In comparison with the leveling of the prior-art process, the skin'pass rolling or temper rolling is highly effective in preventing the stretcher strain. The material after the skin-pass rolling'can be let to stand for several weeks till the pressforming step which is conducted by following the general procedure used to produce shadow masks.

The shadow mask formed via the skin-pass rolling as in the foregoing example is lessened in the amount of thermal deformation even in the presence of a local temperature rise, for a reason stated below.

As heretofore described, FIGS. 3a3d are diagrams showing the residual stress distributions of the material in the respective steps of this invention.

In general, when'a material is rolled and formed into a certain sheet thickness, residual stresses of tension and eompressionare respectively produced at both the surfacelayers of the sheet and the central part thereof as illustrated in FIG. 3a. Upon the subsequent annealing ofthe material, the residual stresses become zero.

When the sheet steel is thereafter subjected to skinpass rolling to work only the surface layer parts, the state of the residual stresses changes and a residual tension distribution, with compression at both the surface layer parts andtension at the central layer part occurs as illustrated in FIG. 3b.

The steel in this state is especially suitable as the sheet steel for the shadow mask in comparison with the steel in the prior art shown in FIG. 3a.

The sheet steel after the skin-pass rolling as shown in FIG. 3b is further subjected to the etching operation by which, as has hitherto been performed, the holes are formed so as to be larger on one side (fluorescent screen side) than on the other side (electron gun side). Then, as shown in FIG. 30, the residual stresses change to a distribution which consists of a large compression on the smaller aperture side and zero compression and a tension over from thesurface to the central part on the larger aperture side. In general, the sizes of the holes'arethe same conventionally produced by etching. More particularly, when the hole pitch is equal to 0.6 mm. the effective sizes of the holes range from 0.2 to

Even when .the material in this state is press-formed and worked as illustrated in FIG. 3d, the tendency of the stress distribution of the material is sustained owing to a low degree of working. Bending moments in the directions of arrows M act at all times.

The shadow mask with such bending moments exerted thereon is under the action of a force of suppressing expansion and therefore diminishes in the amount of thermaldeformation even at the application of the local temperature rise. Where-the etching rate on the larger aperture side is made greater than on the smaller aperture side at the post-etching after the forming as has hitherto been well known, the residual stress distribution is intensified, and hence, the effect of diminishing the thermal deformation is increased.

The range of the reduction (in height) of the skinpass rolling is prescribed by conditions in the working to be set forth hereunder.

The lower limit of the reduction is prescribed, depending on whether the yield point elongation can be decreased or not. In view of the relationship between the reduction and the yield point elongation as illustrated in FIG. 4, a reduction value of at least 0.5% (desirably, at least 1.0%) is considered to be suitable. When the reduction over the lower-limit value is applied, the appearance of the stretcher strain is preveritable.

The upper limit of the reduction is prescribed by the ductility of the material. The relationship between the reduction and the uniform strain as well as the total strain is illustrated in FIG. 5. Since a ductility of several percent is required at the press-forming operation, the upper limit of the reduction is approximately 15%.

While the temperature and'period of the annealing before the skin-pass rolling are not restricted to the values of 600C. and 20 minutes given the previous embodiment, the temperature need not be raised to 900C. which is the temperature shown by the prior art. As stated previously, the annealing step is for the purpose of increasing the ducitlity of the material. It need be carried out to the extent that the crystal grain does not become large (larger than the ASTM grain size 7 or so) and that the unevenness of the sheet as caused by the annealing can be leveled by the skin-pass step.

As regards the annealing temperature, the lower limit is to the extent of the recrystallization temperature. It is usually suitable to perform the annealing in a low temperature range of 550C. 650C.

:As for the annealing period, a value of approximately 10 minutes to approximately 8 hours suffices.

These are effective because, if only a sufficient ductility is attained, even a large yield point elongation can be perfectly erased by the skin-pass rolling step.

According to this invention as stated above in detail, the following effects are achieved:

1. By the application of the skin-pass rolling, a sheet steel material free from a high yield point elongation and slow in the progress of aging can be supplied to the press-forming step.

2. According to the working process of this invention, the press-forming operation is done without di storting the pattern of the holes formed by the etching on account of the annealing and leveling steps, and hence, any non-conforming of the resultant article, due to the damage of the pattern does not arise at all. Further, since the deflection of the pattern is not caused, the production process of the color TV Braun tubes using these shadow masks is sharply improved.

3. The material which does not differ in the yield point from the prior-art material as illustrated at C in FIG. 2 and which is easily worked and hardened, is press-formed. Therefore, the uniformity in deformation is enhanced, and the forming precision is enhanced.

4. For the same reason as in Item 3, the shadow mask after the working is raised in stiffness when hardened, and suffers less from damage in handling.

5. The bending moments can be imparted to the sheet by making use of the distribution of the residual stresses generated by the skin-pass rolling, so that the color shading or deflection on the television picture frame as attributed to the thermal deformation of the shadow mask can be lessened.

' It will be understood that the shadow mask to which this invention is directed is not restricted in the shape of the holes to being Circular, but the above effects are similarly achieved for a rectangular shape or any other shape.

It will also be understood that acceptable yield point elongation, the ductility of the sheet steel and acceptable slow rate for aging for purposes of this invention are respectively 1% yield point and less, 15% ductility and more in total strain, and at least more slow rate of aging than that of leveling (at room temperature for 1.5 months to 1% strain).

Finally, it will be appreciated from the foregoing description that the starting material of the sheet steel is a low carbon steel containing 0.1% or less by weight of carbon and that the thickness of the steel sheet is that suitable for forming shadow masks, i.e., from 0.1 to 0.3

While we have shown and described several embodiments in accordance with the present invention, it is understood that the same is not limited thereto but is susceptible of numerous changes and modifications as known to a person skilled in the art, and we therefore do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are obvious to one of ordinary skill in the art.

What is claimed is:

1. A method for manufacturing a shadow mask, comprising a first step of annealing a material of low-carbon sheet steel, a second step of thereafter subjecting the annealed sheet steel to skin-pass rolling for a reduction of 0.5 l5% in thickness thereby providing a predetermined thickness, a third step of thereafter forming holes in the sheet steel, and a fourth step of thereafter pressforming the sheet steel into a desired plane.

2. The method of claim 1, wherein said first step comprises a step of annealing at a low temperature a material of low-carbon sheet steel subjected to cold rolling.

3. The method of claim 2, wherein the annealing is conducted from 550 650C. for a period of from minutes to 8 hours under an inert gas.

4. The method of claim 1, wherein said second step comprises a step of subjecting said sheet steel to the skin-pass rolling of the reduction of 0.5 to bring the sheet steel into the predetermined thickness and to give rise to a residual stress distribution which consists of compressions at both surface layer parts of said sheet steel and a tension at a central layer part thereof.

5. The method of claim 1, wherein said third step comprises a step of perforating said sheet steel by etching by Which the holes are so formed as to be larger on one surface side than on the other surface side, thereby giving rise to a residual stress distribution which consists of a great compression on the smaller hole side and null stress and a tension over from the surface on the larger hole side to said central layer part.

6. The method of claim 1, wherein the sheet steel has a carbon content of not more than 0.1% and an initial thickness ranging from 0.1 to 0.3 mm.

7. A method for manufacturing a shadow mask, comprising the step of annealing at a low temperature a material of low-carbon sheet steel subjected to cold roll ing, the step of thereafter subjecting the sheet steel to skin-pass rolling of a reduction of 0.5 15% to thus bring it into a predetermined thickness and give rise to a residual stress distribution which consists of compressions at both surface layer parts of said sheet steel and a tension at a central layer part thereof, the step of thereafter forming holes in said sheet steel by etching wherein said holes are made larger on one surface side than on the other surface side of said sheet steel, thereby giving rise to a residual stress distribution which consists of a great compression on the smaller hole side and null stress and a tension over from the surface on the larger hole side to said central layer part, and the step of thereafter press-forming the sheet steel into a desired curved plane.

8. The method of claim 7, wherein the sheet steel has a carbon content not more than 0.1% by weight and a thickness of from 0.1 to 0.3 mm.

9. The method of claim 8, wherein annealing is conducted at 550C. to 650C. for a period of from 10 minutes to 8 hours under an inert gas.

10. The method of claim 1., wherein the sheet steel has a carbon content of not more than 0.1% and an initial thickness ranging from 0.1 to 0.3 mm; the annealing is conducted from 550 650C. for a period of from 10 minutes to 8 hours under an inert gas; and the holes are formed in the sheet steel by etching.

11. The method of claim 1, wherein the desired plane formed by press-forming said sheet steel is a curved plane having bending moments exerted thereon which suppress expansion. 

1. A METHOD FOR MANUFACTURING A SHADOW MASK. COMPRISING A FIRST STEP OF ANNEALING A MATERIAL OF LOW-CARBON SHEET STEEL, A SECOND STEP OF THEREAFTER SUBJECTING THE ANNEALED SHEET STEEL TO SKIN-PASS ROLLING FOR A REDUCTION OF 0.5 - 15% IN THICKNESS THEREBY PROVIDING A PREDETERMINED THICKNESS, A THIRD STEP OF THEREAFTER FORMING HOLES IN THE SHEET STEEL, AND A FOURTH STEP OF THEREAFTER PRESS-FORMING THE SHEET STEEL INTO A DESIRED PLANE
 2. The method of claim 1, wherein said first step comprises a step of annealing at a low temperature a material of low-carbon sheet steel subjected to cold rolling.
 3. The method of claim 2, wherein the annealing is conducted from 550* - 650*C. for a period of from 10 minutes to 8 hours under an inert gas.
 4. The method of claim 1, wherein said second step comprises a step of subjecting said sheet steel to the skin-pass rolling of the reduction of 0.5 - 15% to bring the sheet steel into the predetermined thickness and to give rise to a residual stress distribution which consists of compressions at both surface layer parts of said sheet steel and a tension at a central layer part thereof.
 5. The method of claim 1, wherein said third step comprises a step of perforating said sheet steel by etching by which the holes are so formed as to be larger on one surface side than on the other surface side, thereby giving rise to a residual stress distribution which consists of a great compression on the smaller hole side and null stress and a tension over from the surface on the larger hole side to said central layer part.
 6. The method of claim 1, wherein the sheet steel has a carbon content of not more than 0.1% and an initial thickness ranging from 0.1 to 0.3 mm.
 7. A method for manufacturing a shadow mask, comprising the step of annealing at a low temperature a material of low-carbon sheet steel subjected to cold rolling, the step of thereafter subjecting the sheet steel to skin-pass rolling of a reduction of 0.5 - 15% to thus bring it into a predetermined thickness and give rise to a residual stress distribution which consists of compressions at both surface layer parts of said sheet steel and a tension at a central layer part thereof, the step of thereafter forming holes in said sheet steel by etching wherein said holes are made larger on one surface side than on the other surface side of said sheet steel, thereby giving rise to a residual stress distribution which consists of a great compression on the smaller hole side and null stress and a tension over from the surface on the larger hole side to said central layer part, and the step of thereafter press-forming the sheet steel into a desired curved plane.
 8. The method of claim 7, wherein the sheet steel has a carbon content not more than 0.1% by weight and a thickness of from 0.1 to 0.3 mm.
 9. The method of claim 8, wherein annealing is conducted at 550*C. to 650*C. for a period of from 10 minutes to 8 hours under an inert gas.
 10. The method of claim 1, wherein the sheet steel has a carbon content of not more than 0.1% and an initial thickness ranging from 0.1 to 0.3 mm; the annealing is conducted from 550* - 650*C. for a period of from 10 minutes to 8 hours under an inert gas; and the holes are formed in the sheet steel by etching.
 11. The method of claim 1, wHerein the desired plane formed by press-forming said sheet steel is a curved plane having bending moments exerted thereon which suppress expansion. 